Superconducting magnets are well known and used for producing very high strength magnetic fields in applications such as magnetic resonance imaging. The superconducting magnet is typically made up of a number of coils of superconducting wire which, in use, must be held at cryogenic temperatures typically of the order of 4K, the boiling point of liquid helium. Alternatively, systems using so-called high temperature superconductors are known. These may operate at the temperature of boiling neon, hydrogen or nitrogen. Critical temperatures in the region of 18-80K are typical for such materials. While the present invention will be described with particular reference to systems operating at about 4K, which are cooled by boiling helium, the present invention may be applied to high temperature superconducting arrangements.
A typical arrangement has superconducting coils immersed in a bath of liquid cryogen, for example helium. The liquid cryogen is allowed to boil, and maintains the coils at a steady temperature of its boiling point, about 4K in the case of helium. A recondensing refrigerator is provided, and provides cooling by recondensing the boiled-off cryogen vapour back into a liquid. Thus, the overall consumption of liquid cryogen is reduced to a low value, since most if not all of the boiled off cryogen is recondensed back into liquid form within the cryogen vessel. Cryogens other than helium may of course be used, depending on the cryogenic temperature required.
FIG. 1 shows a cross-section of a former 10 and superconducting coils 12 mounted thereon, typical of a solenoidal superconducting magnet for MRI or NMR imaging systems. The former 10 and coils 12 are housed within a cryogen vessel 14. The cryogen vessel 14 is typically partially filled with a liquid cryogen, not shown in the drawing. The cryogen vessel itself is housed within an outer vacuum chamber 16, and the space between the cryogen vessel 14 and the outer vacuum chamber 16 is evacuated. A thermal shield 18 is typically also provided, in the evacuated space between the cryogen vessel and the outer vacuum container. This shield reduces the incidence of radiated heat from the outer vacuum chamber which may reach the cryogen vessel. A service neck 20 is typically included. In operation, this neck may house a recondensing refrigerator.
A difficulty arises when the system is in transit, awaiting installation. Typically, the system is transported filled with liquid cryogen, but the recondensing refrigerator is unable to operate due to the absence of a suitable power source, or due to regulatory restrictions. During the transit time, the cryogen is allowed to boil, keeping the coils 12 at the required cryogenic temperature. The cryogen thus acts as a thermal battery. Service neck 20 provides an escape path for boiled off cryogen to leave the cryogen vessel 14. The boiled off cryogen is allowed to vent to atmosphere. The system may be required to be capable of remaining in this boiling thermal battery state for a duration of up to about 30 days. When a cryogen such as helium or neon is used, the cost of the cryogen lost by boiling may become significant. When hydrogen is used a cryogen, an explosion risk may result.
It is required to keep the coils at a cryogenic temperature, since otherwise the commissioning of the system on installation becomes expensive, difficult and time consuming. If the system has heated up to ambient temperature, which will happen if the liquid cryogen boils dry, the system must be cooled and refilled with the relatively costly liquid cryogen before being commissioned. In some regions of the world, it is very difficult to obtain the large supplies of the liquid cryogen required for such an operation if not planned for in advance. Such a re-cooling and refilling is also time consuming, and costly both in terms of the time a field engineer must spend on site installing the system, and the material cost of the cryogen used.
In magnet systems such as that illustrated in FIG. 1, the volume of the cryogen tank 14 is largely defined by the minimum allowable shipping time. To allow a shipping time with the recondenser inoperable of, say, thirty days, a large volume of liquid cryogen must be accommodated in the system to ensure that it does not run dry during transit. The volume of this required cryogen reservoir plays a large part in determining the final size of the whole system.